Modulated corner reflector



March 7,1967

VOLTAGE B. L. LEWIS MODULATED CORNER REFLECTOR VARIABLE COMPONENTS STATION 2 Filed March 31, 1966 VOLTAGE SOURCE STATION INVENT OR BERNARD L. LEWIS ATTORNEKS United States Patent Oflice 3,363,464 Patented Mar. 7, 1967 3,308,464 MODULATED C(DRNER REFLECTOR Bernard L. Lewis, Satellite Beach, Fla, assignor to Radiation Incorporated, Melbourne, Fla, a corporation of Florida Filed Mar. 31, 1966, Ser. No. 539,078

. 8 Claims. (Cl. 343 l8).

The present invention relates generally to corner reflectors, and more particularly to corner reflectors adapted to provide modulatable retrodirective reflectivity of RF radiation incident thereon.

As is well known, the conventional corner reflector, of either the metal mirror or wire type, is useful for increasing the directivity of a radiating system, for providing a radar target, for ground markers, and for a variety of other purposes in communications and telemetry links. It has been suggested that the limiting angles of such a reflector may be widened by providing a solid dielectric medium of greater dielectric constant that that of air in the corner formed by the metal surfaces. It has also been suggested that a ray refracted by the solid dielectric medium filling the corner of the polyhedral configuration formed by the conductive surfaces of the reflector may be totally or partially reflected from a dielectric-air interface, in accordance with Snells law, such an interface provided by dispensing with one of the conducting reflector walls.

In particular, it has been found that the use of a dielectric filler, within the region formed by the metal walls of the corner reflector, having an index of refraction greater than unity (the index of refraction for air) provides retrodirective action over a much larger solid angle than can be obtained with a conventional corner. This extended coverage may be explained as follows: If a ray incident on the dielectric face of the filler material makes an angle of incidence i with the normal to that face, it is refracted in passing through the surface and travels through the dielectric material at an angle of refraction r to the normal, where r is less than i as given by the equation I With respect to the axis of symmetry of a conventional triangular or trihedral corner reflector, for example, the normal angtlar range of ray incidences capable of being handled by such a reflector is approximately :45 degrees to that axis. By appropriate selection of the dielectric constant (and thus the index of refraction) of a dielectric filler it is possible for the corner reflector to provide extended coverage for rays incident over the range of :90 degrees to the normal to the dielectric sur face, and by reference to Equation 1, above, it will be noted that this specific result is achieved where n is greater than or equal to 1.41.

Such devices have the advantages of extremely wide angular coverage, simplicity, economy, adaptability to flush mounting, durability, capability of withstanding high temperatures and pressures, and suitability for target or aircraft radar reflectivity augmentation as well as foruse at optical frequencies. If the corner reflector is constructed solely of dielectric material, that is, withoutusing any metallic walls, as opposed to simply using a dielectric filler or dispensing with a single metallic wall, still further advantages are obtained by way of increased efliciency and by virtue of the fact that the corner reflectivity is modulatable.

Accordingly, it is an object of the present invention to provide an improved dielectric corner reflector.

It is another object of the present invention to provide a modulated corner reflector.

Briefly, according to the present invention the corner reflector is constructed of a dielectric material in the shape of a polyhedron providing a corner such as a dihedral, trihedral (i.e. triangular trihedral, or square (i.e. square trihedral) reflective corner. The dielectric constant of the material is preselected to produce retrodirective reflection from the corner at an angle of reflection equal in both sign and magnitude to the angle of incidence of the radiation in question (alternatively termed waves, rays, or beams in succeeding portions of the specification) by use of total internal reflection criteria at interfaces between the dielectric of which the corner is composed and the medium in which the corner is immersed (hereinafter also called the environmental medium).

Modulated retrodirective reflectivity is achieved by depositing or otherwise attaching an interconnected array of electricalcomponents having one or more voltage variable parameters on or to one or more of the faces of the dielectric corner, and by driving or exciting the electrical components with a modulating signal applied via parallel conductors printed on the respective face in a direction normal to the E field of the incident radiation (for example, RF waves) to which the corner reflector is arranged to be responsive. 5 In this manner, varying voltages applied to the modulation input lines are effective to correspondingly vary the voltage variable parameter of the deposited component array and, thereby, to change the RF reflection coefiicient of the di electric face involved. Thus, modulated specular reflection of the incident carrier may be provided for the transmission of information or intelligence from a station at which the corner reflector is located, or with which the reflector is associated to a remote station via the communication or telemetry link therebetween and utilizing a carrier transmitted by the remote station.

Therefore, it is still another object of the present invention to provide a dielectric corner reflector having reflecting surfaces whose respective RF reflection coeflicients can be varied to modulate the frequency of an RF carrier incident upon the corner reflector.

A further object of the invention is to provide a modulatable dielectric corner reflector wherein modulation of an RF signal is achieved by varying the excitation voltage applied to an array of voltage variable electrical components printed on one or more of the reflector faces, to correspondingly vary the RF reflection coeflicient of the respective face.

A further aspect of the present invention resides in a modulation relay system utilizing a nest or retrodirective array of modulatable dielectric corner reflectors of the aforementioned type. According to this aspect of the invention a plurality of remote stations having transmitting and receiving capabilities may communicate with one another by modulating the information to be transmitted to the other stations on a distinct subcarrier frequency modulating a high frequency carrier which is at all times directed toward the nest of corner reflectors. The relay station at which this nest is located is provided with a broadband antenna or antennas, also arranged to receive the high frequency carrier, and a broadband receiver by which to detect the senders subcarrier. This subcarrier, on which the information is impressed, is employed to modulate the reflection coeflicient of any or all corner reflectors in the nest whereby to modulate the respective carrier of the station to which the information is to be directed. Each receiving station may then heterodyne the received carrier with its transmitted carrier to recover the sending stations subcarrier and thence detect the transmitted information therefrom. Since each station receives its own transmitted high frequency carrier via a separate corner reflect-or, that carrier is of significance only to the particular station which may be transmitting at any given time.

Accordingly, it is still another object of the present invention to provide an improved modulation relay system utilizing dielectric corner reflectors of the aforementioned type.

Yet another object of the invention is to provide a relay for a communications link, wherein the relay comprises a retrodirective array of modulatable dielectric corner reflectors arranged to be illuminated by high frequency carrier signals from a plurality of remote transceiver stations.

The above and still further objects, features and attendant advantages of the present invention will become apparent from a consideration of the following detailed description of specific embodiments thereof, especially when taken in conjunction with the accompanying drawings, in which:

FIGURE 1 is a perspective view of a solid dielectric (polyhedron) corner reflector in accordance with the present invention;

FIGURE 2 is a top view of the corner reflector of FIGURE 1 showing the path of a ray incident thereon;

FIGURE 3 is a perspective view of a solid dielectric triangular trihedral (polyhedron) corner reflector with a voltage variable component array, shown in functional block diagrammatic form, printed on one of the reflecting faces thereof; and

FIGURE 4 is a diagram of a modulation relay system employing a retrodirective array of corner reflectors in accordance with the invention.

Referring now to FIGURES 1 and 2, the corner reflector 10 is a solid dielectric mass in the shape of a polyhedron. Purely for the sake of example, corner reflector 10 is illustrated as having a 90-degree dihedral configuration formed by reflecting sides 12. and 14, and having a base (aperture) 17. It will be appreciated, however, that the reflector may take a shape analogous to that of a conventional metallic plate or Wall reflector, such as a triangular trihedral or a square trihedral (square corner), with the exception, of course, that corner reflectors according to the present invention consist of a solid dielectric material.

The device operates in accordance with the principle of total internal reflection at interfaces between the high dielectric constant material of which it is formed and the relatively low dielectric constant medium, such as air or vacuum, within which it is placed. Since the reflection coeflicient (ratio of reflected wave to incident wave) is equal to unity when the angle of incidence i of the incoming radiation is greater than the critical angle (the angle between the normal to the dielectric interface on which the ray is incident and the ray itself, and at angles in excess of which the ray is subjected to total internal reflection), it will be observed that the dielectric corner reflector is capable of high efiiciency operation to direct incident radiation back in the direction from which it arrived (i.e., the reflector is retrodirective). As will be explained more fully presently, corner reflector is capable of producing angles of reflection equal in both sign and magnitude to respective angles of incidence over as much as a 180-degree angle of coverage, i.e., within the range from i90-degrees with respect to the axis of symmetry of the corner.

The principle of operation of the dielectric corner reflector will be better understood by reference to FIG- URE 2. Assume that a ray AB is incident upon face 17 at an angle 0 as shown. The ray is refracted upon entering the dielectric material of the corner, following the path BC at an angle of refraction a with respect to the normal to face 17, Where 0 ,=arc sin E ray BC is totally reflected as ray CD and is incident on dielectric interface 14 at an angle 'y=45--oc, since the angle of reflection is equal to the angle of incidence.

Again, if 'y is greater than 0 ray CD is totally reflected as ray DE and strikes the entrance face 17 at an angle a. If a is less than 0 the ray will pass through interface 17 and make an angle of refraction of 6 with respect to the normal to that interface, as shown. Thus, the refracted ray EF leaves the dielectric corner reflector in the direction from which incident ray AB arrived.

Since, from expression 2 and the values of angles [3 and 'y in terms of or as given above the foregoing reasoning will demonstrate that to provide this operation, the dielectric constant of corner reflector It) must be such that so that the reflector will function retrodirectively over the full -degree angle permitted by the geometry. A solution of expression 6 indicates that 6 must be greater than 6.4, corresponding to an index of refraction of 2.52. While a smaller value of dielectric constant would be usable, it would not admit of retrodirective action over the full 90 solid angle with respect to the axis of symmetry of' the corner. On the other hand, retrodirective action is obtainable over solid angles of greater than 90 degrees by use of dielectrics having an index of refraction greater than 2.52. Since refraction of waves at the front face (e.g., 17) increases the angle of incidence on the reflecting faces, a coverage angle approaching degrees is possible with proper selection of dielectric constant. It will be appreciated that considerations similar to those which have been described with reference to the dihedral corner are applicable to polygonal dielectric corner reflectors of other geometry. For the dihedral corner an index of refraction of 2.52 can be obtained by use of a dielectric such as rutile, sphalerite (ZnS), strontium titanate, arsenic trisulfide, glass amorphous selenium, germanium, or pyrite, to name a few of the suitable materials.

The advantages of a dielectric corner reflector include, in addition to high efliciency, retrodirectivity over an extremely wide angle of coverage, simplicity and economy, the fact that the reflection coefficient of such a corner is more readily modulated than that of other reflecting devices. According to the present invention such modulation may be achieved, for example, by use of the embodiment of FIGURE 3. A dielectric polygon 21 forming a triangular trihedral corner with reflecting faces 24, 25, 26, is modified by depositing on one or more of the plane reflecting faces thereof for example face 26, an array of electrical elements 29 each having a voltage variable parameter. Deposition of the elements 29 and of conductive lines such as 32 and 34 across which the voltage variable elements are bridged may be performed in any convenient and conventional manner, such as by vacuum deposition, printed circuit techniques, or the like.

Elements 29 may, for example, comprise an array of voltage variable capacitors, diodes, or transistors, to which the desired signal is applied via lines 32 and 34 from a modulating voltage source 37. The voltage applied to the modulation input lines produces a corresponding variation of the capacitance of the capacitors or the conductivity of the diodes or transistors to change the R-F reflection coefficient of the face or faces upon which these voltage variable elements are disposed. In the case of active elements, such as transistors, power bus lines coupled to an appropriate biasing supply, as well as modulation bus lines connected to the modulating source, are required to permit the desired variation of the R-F reflection coefficient. As the reflection coefficient of one or more of the reflecting faces of the corner is varied, the reflected wave is modulated accordingly with the information desired to be returned to the station from which the incident signal, e.g., an R-F carrier Wave, is being transmitted. The parallel conductors between Whieh the voltage variable elements are connected are preferably arranged normal to the R-F E field of the incoming Wave so that modulation of that wave is effected only as a result of the variation of reflection coefficient of the reflecting dielectric interface between the corner and the surrounding or environmental medium.

It is to be noted that the use of voltage variable elements by which to vary reflection coefficient is applicable to corner reflectors in which the remaining reflecting faces are covered with conductive sheets or walls, as well as to corners where the remaining reflecting faces consist solely of dielectric interfaces.

The corner reflector of FIGURE 3 may be modulated with frequencies from DC. to the megacycle range and provides equivalent scattering cross-sections of thousands of square meters.

Modulatable corner reflectors of the type shown in FIGURE 3 may be arranged in a nest or array at a central station or site to provide a relay system for use in communications links. Such a system is shown in diagrammatic form in FIGURE 4. The site or station 50 at which the nest 52 of corner reflectors is located includes one or more broadband antennas 56 for receiving information transmitted from one or more remote stations, designated stations 1, 2, n, of the communications link. Antenna 56 is coupled to a video receiver 59 which may be single or multiple channel with or Without R-F gain stages and conventional in all respects. The receiver 59 output is employed to control the reflection coeflicient of the reflecting faces of the corner reflectors in nest or array 52 in the manner previously de scribed in connection with FIGURE 3.

In the operation of the system of FIGURE 4, all of the remote stations are provided with conventional high frequency transmitting and receiving equipment so that each may transmit a carrier which can be modulated with a sub-carrier upon which desired information modulation may be impressed, and receive signals reflected from the corner reflector array site 50. The transmitting and receiving antennas of the remote stations (des ignated 62-1, 62-42, 62-21) are directive and arranged to illuminate the corner reflector nest at the central site such that the signal transmitted by each station is reflected directly back to that station. When information is desired to be transmitted from one of the remote stations, its carrier is modulated with a preselected sub-carrier and the latter, in turn, with the information to be sent. The carrier is picked up at receiving antenna 56 and fed to receiver 59 which is operative to detect the sub-carrier. The output of the receiver detector is applied to the modulation bus lines connected to the volt- 6 age variable elements disposed on selected reflecting faces of the corner reflectors to vary the reflection coeflicient and thereby to modulate the carrier of each of the other stations with the sending stations sub-carrier. Since each station receives its own carrier via reflection from nest 52, the sending stations sub-carrier may be recovered at any station by zero beating the received signal With its own transmitting signal. The sub-carrier is then demodulated to recover the transmitted information. It should be noted that the R-F carrier of each station need not be of the same frequency. In this manner, the R-F carrier frequency is of interest only to. the station transmitting that frequency since any information transmitted via the communication link is received in the form of a signal modulating that carrier from the relay comprising the corner reflector nest.

While I have described and illustrated one specific embodiment of my invention, it will be clear that variations of the details of construction which are specifically illustrated and described may be resorted to without departing from the true spirit and scope of the invention as defined in the appended claims.

I claim:

1. A retrodirective corner reflector for modulating R-F waves reflected therefrom, said corner reflector comprising a polyhedron composed of dielectric material having a preselected dielectric constant greater than that of the surrounding medium in which said reflector is to be used, said polyhedron having a plurality of intersecting faces forming the corner from which incident R-F Waves are to be reflected; a source of modulating signal; and means, disposed on at least one of said intersecting faces, responsive to modulating signal generated by said source to vary the R-F reflection coeflicient of said at least one face in accordance therewith, whereby to modulate R-F waves reflected from said corner with said signal.

2. The combination according to claim 1 wherein said means comprises an array of interconnected elements, each having a signal-variable electrical parameter, deposited on said at least one face; and means connecting said array of interconnected elements to said source.

3. The combination according to claim 2 wherein the remaining ones of said intersecting faces are covered with conductive sheet.

4. The combination according to claim 2 wherein each of said elements is a voltage variable capacitor.

5. The combination according to claim 2 wherein each of said elements is a transistor.

6. The combination according to claim 2 wherein each of said elements is a diode.

7. A modulatable relay system for a communications system having a plurality of stations adapted to transmit and receive information via R-F carrier waves, each of said stations having an assigned sub-carrier frequency for modulation of the respective R-F carrier and on Which the information to be transmitted is to be impressed, said relay system comprising a receiver; a broadband antenna for feeding R-F carrier waves transmitted by one or more of said stations to said receiver, said receiver including means for detecting said sub-carrier from the R-F carrier wave applied thereto; an array of retrodirective corner reflectors arranged to be illuminated by the R-F carrier waves transmitted by said plurality of stations and for reflecting the respective R-F carrier wave back to the transmitting station, each of said corner reflectors comprising a polyhedron composed of dielectric material having a preselected dielectric constant greater than that of air, each polyhedron having a plurality of intersecting faces forming the corner from which incident R-F waves are to be reflected, and means disposed on at least one of said intersecting faces of each of said polyhedrons for varying the R-F reflection coefficient of the respective face in response to the application of electrical signal thereto; and means for applying sub-carrier detected by said receiver to said means for varying reflection coefficient, whereby to modulate RF carrier waves reflected by said array of corner reflectors with the applied sub-carrier and, thereby, with any information impressed on the last-named sub-carrier.

8. The combination according to claim 7 wherein said means for varying reflection coefficient comprises an &

array of interconnected elements, each having a voltage variable electrical parameter.

No references cited.

5 CHESTER L. JUSTUS, Primary Examiner.

D. C. KAUFMAN, Assistant Examiner. 

7. A MODULATABLE RELAY SYSTEM FOR A COMMUNICATIONS SYSTEM HAVING A PLURALITY OF STATIONS ADAPTED TO TRANSMIT AND RECEIVE INFORMATION VIA-R-F CARRIER WAVES, EACH OF SAID STATIONS HAVING AN ASSIGNED SUB-CARRIER FREQUENCY FOR MODULATION OF THE RESPECTIVE R-F CARRIER AND ON WHICH THE INFORMATION TO BE TRANSMITTED IS TO BE IMPRESSED, SAID RELAY SYSTEM COMPRISING A RECEIVER; A BROADBAND ANTENNA FOR FEEDING R-F CARRIER WAVES TRANSMITTED BY ONE OR MORE OF SAID STATIONS TO SAID RECEIVER, SAID RECEIVER INCLUDING MEANS FOR DETECTING SAID SUB-CARRIER FROM THE R-F CARRIER WAVE APPLIED THERETO; AN ARRAY OF RETRODIRECTIVE CORNER REFLECTORS ARRANGED TO BE ILLUMINATED BY THE R-F CARRIER WAVES TRANSMITTED BY SAID PLURALITY OF STATIONS AND FOR REFLECTING THE RESPECTIVE R-F CARRIER WAVE BACK TO THE TRANSMITTING STATION, EACH OF SAID CORNER REFLECTORS COMPRISING A POLYHEDRON COMPOSED A DIELECTRIC MATERIAL HAVING A PRESELECTED DIELECTRIC CONSTANT GREATER THAN THAT OF AIR, EACH POLYHEDRON HAVING A PLURALITY OF INTERSECTING FACES FORMING THE CORNER FROM WHICH INCIDENT R-F WAVES ARE TO BE REFLECTED, AND MEANS DISPOSED ON AT LEAST ONE OF SAID INTERSECTING FACES OF EACH OF SAID POLYHEDRONS, FOR VARYING THE R-F REFLECTION COEFFICIENT OF THE RESPECTIVE FACE IN RESPONSE TO THE APPLICATION OF ELECTRICAL SIGNAL THERETO; AND MEANS FOR APPLYING SUB-CARRIER DETECTED BY SAID RECEIVER TO SAID MEANS FOR VARYING REFLECTION COEFFICIENT, WHEREBY TO MODULATE R-F CARRIER WAVES REFLECTED BY SAID ARRAY OF CORNER REFLECTORS WITH THE APPLIED SUB-CARRIER AND, THEREBY, WITH ANY INFORMATION IMPRESSED ON THE LAST-NAMED SUB-CARRIER. 